Heterochromatin: a rapidly evolving species barrier

Abstract

Recent work has shown that changes in the sequence composition of heterochromatin, or in the factors that maintain that heterochromatin, may play an important role in speciation.

Figure 1. DNA can be divided into euchromatin and heterochromatin.

Shown is a representative acrocentric chromosome containing both condensed heterochromatic (dark gray) and less condensed euchromatic regions (light gray). Beside each region are characteristics typical for each type of chromatin.

Figure 2. A model of how the mishandling of a specific heterochromatic region might cause lethality in female hybrid embryos.

This model is based on the results of the article published by Ferree and Barbash in this issue of PLoS Biology . A cross between D. melanogaster males and D. simulans females, which results in hybrid females that die early in embryogenesis, is shown on the right. The drawing on the left depicts a cross between D. melanogaster males to D. melanogaster females for comparison. For simplicity, only the X chromosomes are shown and the heterochromatic region is specified with a darker color. In both crosses, the fusion of the sperm and egg results in zygotes carrying a pair of X chromosomes. The cross with the D. melanogaster females leads to normal chromosome segregation during anaphase of mitotic divisions 10–13 in female embryos. In the cross with D. simulans females mitosis fails to be completed normally in the hybrid female embryos. While the maternal X chromosomes segregate normally towards opposite spindle poles, the segregating centromeres of the paternally derived X chromosomes are connected by a bridge of chromatin. This bridge, which is heterochromatic and comprised of a region rich in the 359-bp repeat, causes improper segregation of the sister chromatids of the X chromosome, an event that eventually leads to aberrant mitotic divisions and ultimately the death of female hybrid embryos. The lagging or bridging of the 359-bp region is likely due to an absence of a maternally loaded factor in the D. simulans egg (shown as yellow diamonds in the D. melanogaster egg). We imagine that this factor might be involved in the resolution of heterochromatic threads that have been shown to connect the pericentromeric regions of both mitotic and meiotic chromosomes in normal cells (see text for a description). The absence of this factor prevents the proper formation or maintenance of chromatin structure in the 359-bp repeat region in female hybrid embryos.